When To Start Tomato Seeds Indoors By Zone

Tomato lovers probably want to get a head start on the growing season. So, you might be wondering when to start tomato seeds indoors (or more specifically, how early you can start tomato seeds indoors).

Timing the start of your tomato plants is an essential part of growing healthy tomatoes. Knowing how to do it properly within the parameters of your situation will make young plants even healthier.

A lot goes into starting seeds indoors. Assess the amount of space you have, both in the starting station and the final destination for your tomatoes. The frost date is also an important part of the process. The type of tomatoes you choose to grow has a bearing on when to begin, too. 

Let’s discuss when to start tomato seeds indoors by zone, and how to do it with timing in mind. Growing tomatoes gets easier the more you do it, so building a successful foundation for your tomato transplants will make a huge difference in your overall gardening practice. 

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When to Start Tomato Seeds

A general rule of thumb is to start spring vegetable seeds anywhere from 8 to 3 weeks before the last frost. For tomatoes, when you begin depends on the varieties you want to grow and your USDA hardiness zone.

Remember that your mode of starting seedlings (indoors, in a greenhouse, and so on) has a bearing on when you can appropriately transplant. Here, we’ll break down each of these factors to make growing your own tomatoes with the right timing easy.

Starting Tomato Seeds by USDA Hardiness Zone

Most guides say to start tomatoes 6 to 8 weeks before the last frost date. However, this varies depending on the region you live in, and its associated USDA growing zone.

For simplicity, we’ve created a list of zones and planting times. This is highly generalized, and the rest of this piece will address the nuance involved in starting and caring for tomato seedlings.

Heat-loving plants are going to have no spring frost date to contend with in the tropics, and people there may be able to start tomato seedlings outdoors at multiple points in the year. Those in more arctic climates need to have their seeds sown indoors in a space where they aren’t subject to the harsh cold. 

Here is a list of USDA hardiness zones and when to start tomatoes. To determine the time to start tomatoes in the southern hemisphere, refer to Worldwide Hardiness Zone maps:

Areas with longer temperate seasons have more transplant timing options. In tropical regions, there are two very long growing seasons, once in summer, and again in fall.

In zone 8b, where I live, tomato growers have two fruiting phases: once in spring, and again in early fall. We spend our time in the intense summer heat keeping our tomato plants alive for a second harvest.

The average last frost date isn’t the only way to determine when to start, though. So, let’s tackle those factors below. 

Tomato Seed Starting Methods

The mode you choose for starting seeds indoors has an impact on how successfully your seedlings grow. Thankfully, winter gives most of us time to browse a seed catalog, or look at each seed package we’ve stored away for spring.

Those in more temperate or tropical regions might have to intentionally take a break to have the same amount of planning time. That’s because the soil temperature is warm enough for long periods, and warmer soil generally allows for healthier tomatoes. 

One thing is for sure: growing tomatoes from your own transplants with a good root system ties in directly to growing plants that produce great fruit. Here are a few ways to avoid weak seedlings that dampen your head start on the spring season. 

Space

How much space you have is the key to determining how to start. Tomato seed starting is more high maintenance than growing lettuces or kale, for instance. You’ll need a lot of direct sun, and most people starting in winter need a grow light to supplement a small sunny window on the south side of a home.

If you have enough to have room for an outdoor greenhouse that keeps seedlings out of freezing temperatures, you don’t need to clear any space indoors.

Ways to Start

While a window is an adequate option for starting tomato seeds, you’ll need extra light and warmth from a light and heat mat, respectively. This goes for people who start seeds indoors, and for those who start in a greenhouse or grow tent.

A heating mat under the seed trays, pots, or starter blocks helps raise the soil temperature to 20°F more than the surrounding air if needed. Measure the temperature in the room, and adjust the mat controls so the seeds have several hours of 60 to 70°F (16-21°C) heat from below.

A grow light, as we’ve mentioned, is essential. Tomato seed starting requires light for germination. They need at least 8 hours of direct light to develop into sturdy seedlings. On a window sill, use a grow light to supplement, especially where obstructions block out the direct sun, or where the sun is scant to begin with.

South-facing windows are great for those in the northern hemisphere, while north-facing windows work best for those in the southern hemisphere. 

A grow tent gives you an enclosed space where your plants grow in optimal conditions. Like a starting setup with a grow light and heating mat, you can place grow tents anywhere they fit. Tents have easily controlled humidity, temperature, and light conditions that give you wiggle room in terms of timing.

Greenhouses are another indoor option for starting tomatoes. Thermal mass can be your heat source, and greenhouses can also allow for appropriate lighting. Supplement with grow lights as needed.

One awesome aspect of well-stocked greenhouses or grow tents is you’ll have more success growing year-round. Timing isn’t necessary once you get it down. These options are most viable for those with long winters. With climate controls, there are more options as to which tomato varieties you can grow.  

Planting

Let’s do a little run down of the steps for tomato seedling care in conjunction with timing. We’ll discuss sowing seed, caring for sprouts and seedlings, and troubleshooting issues that can arise in the process. 

Pre-Planting

When the time comes to sow your tomato seeds think about a few things.

First, what kind of tomatoes do you want to grow? Are you going to grow determinate tomatoes, which stay smaller? Or do you want to grow indeterminate tomatoes that grow anywhere from 6 to 20 feet and require a trellis?

Another consideration when it comes to tomato varieties is what to do with ripe fruit. Maybe you want to grow sauce tomatoes or bush tomatoes. Or maybe you’d like to grow cherry tomatoes. Maybe heirloom varieties are most attractive to you as a gardener.

Regardless of type, follow the seed packet to some extent to determine timing, spacing, and consider the final destination for your plant. Seed packets often have a little more information about the particular species of tomato you are growing and what the basic needs will be. 

Sowing

The planting and potting accessories you use have a huge impact on the transplant process. Plastic starter pots and trays work, but they break down over time and could promote transplant shock in small seedlings. Peat pots can be planted whole in the garden without disturbance to the root ball.

We highly recommend our Epic 6-cell starting trays. They function just as plastic starter pots do, without the risk of shock, and they last a lifetime. They will ensure healthy transplants.

If you’d like to repurpose materials around your home for starting, yogurt cups are a great alternative. Egg cartons also work for germination. Egg cartons will biodegrade much like peat, but you will need to transplant your seedlings quickly into a larger container to allow for root development.

To start indoors, choose a receptacle and add a seed starting mix. Tamp it down gently. Do not overwhelm the seeds with fertilizer, as too much in the early stages burns plants and prevents germination.

Plant seeds in the 6 to 8 weeks before the last frost. Put two or three seeds in the soil at about ⅛ inch of depth. Cover them at the soil line and press down slightly. Then add enough water to get the soil moist. Keep it moist throughout the germination process.

Transplant Strategies

So you’ve started your tomatoes super early, way outside the 6-8 week window. No problem, as long as proper climate controls exist and you’re willing to up-pot them before they head outdoors.

Don’t plant them outside in cold weather! Instead, take your seedlings and plant them in larger containers (gallon grow bags work) with potting mix amended with tomato food. They can stay indoors too. Care for them as you would outdoor plants, and give them good growing conditions. 

In regular timings, transplant seedlings carefully outside in your raised beds or whatever you use as soon as they are ready. Avoid disturbing the root ball. Place peat containers whole in the garden. The same goes for soil blocks that come from the starter cells we mentioned above.

Tomatoes enjoy deep planting. Make sure the entire stem is covered at the base. Provide adequate spacing between plants so they don’t overcrowd one another. 

Troubleshooting

In the process of working with seedlings, you may encounter some issues. If you start early, you’ll have more time to work out situations that commonly arise when starting plants by seed.

Firstly, if you notice spindly seedlings, you could be watering too much with not enough light, or adding too much light with inadequate water. Because spindly seedlings make spindly plants, make adjustments in light and water quickly so you don’t end up with weak transplants. 

If the air temperature is too cold in your starting area, this could prevent germination. It could also prevent growth in already sprouted seedlings, and potentially create a situation where root rot and damping-off are possible. Heat, therefore, is an essential part of starting tomatoes. Light and heat mat placement, along with the ambient temperature is important. 

If you collect seed from a diseased plant (which can easily happen when the disease is not readily observable) and start it, that could cause damping-off or fungal and bacterial growth. Most seeds from large distributors have quality controls that prevent this, so order from reputable sources. Start questionable seeds away from others to prevent the spread of disease.   

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